The present invention relates to contact lenses and more particularly to soft contact lenses and the design and construction thereof. This invention further relates to lower minus power hydrogel contact lenses.
Contact lenses for correction of ammetropic vision are presently available in a variety of types, styles and materials adapted for correction of most ammetropic conditions. The use of these lenses has presented a number of problems however, as they are worn in intimate contact with the delicate tissues of the cornea of the eye. The special nature of the corneal tissue requires corresponding special considerations in the design, prescription and fitting of these lenses for correction of vision.
The cornea is a nonvascular tissue, because a supply of blood vessels would interfere with the transparency required for proper vision. Yet it is a living tissue and must be provided with oxygen and nutrients, and waste products of metabolism must be removed therefrom. To this end the cornea is bathed in a continuously renewed flow of tear fluid which supplies nutrients and removes metabolic wastes. Oxygen dissolves in the tear fluid directly from the atmosphere to which the eye is exposed, and then diffuses from the tears into the corneal tissue. When the eye is closed for long periods, as during sleep, the amount of oxygen reaching the cornea is reduced, but, even then, enough oxygen ordinarily reaches the cornea through the tears to provide for its metabolic needs.
When a contact lens is fitted to the cornea for correcting vision a thin film of tear fluid remains between the lens and the cornea. However, this thin layer of tears is no longer exposed to the atmosphere for direct absorption of oxygen. Hence some provision must be made for supplying oxygen to the cornea beneath the contact lens either by exchange of the layer between the lens and cornea with fresh, oxygenated fluid or by making the lens permeable to oxygen. Similarly, removal of waste products requires exchange of the tear layer between the lens and the cornea with fresh tear fluid. In practice, the cornea can survive for some time without adequate oxygen supply or waste removal. Accordingly, hard, oxygen-impermeable contact lenses made, such as of poly(methyl methacrylate), typically can be used provided they are worn for periods associated with normal waking hours. Although the cornea can recover partially during the period in which the lens is not worn, short term and long term chronic changes have been reported. Furthermore, some exchange of tear fluid is possible with such lenses due to the generally random movement of the lens over the surface of the cornea as it is pushed to and fro by the eyelids during blinking. This mechanism for the exchange of tear fluid is effective especially when lenses of relatively small diameters are fitted which cover only the central portion of the cornea.
In order to increase patient acceptance, oxygen-permeable hydrogel contact lenses have been developed. These lenses, which are made of a water-swollen synthetic resin, allow at least some oxygen to diffuse through the body of the lens to oxygenate the tear film underneath and thereby to supply oxygen to the cornea upon which they rest. However, thicker hydrogel lenses containing modest amounts of water, such as 25% to 59% by weight of the total weight of the hydrated lens, do not transmit enough oxygen to provide all the needs of the cornea, especially when the eyelids are closed. Accordingly, these lenses also must be removed during periods of sleep. The soft lenses also do not permit significant diffusion of metabolic products, and therefore the removal of these products depends on the exchange of tear fluid produced by movement of the lens. Since soft lenses are designed to cover the entire cornea and since they tend to fit the corneal contour rather snugly, some consideration must be given to the shape of the lens if it is to have adequate movement to produce an adequate exchange of tear fluid.
In order to permit essentially continuous or extended wear of contact lenses, so that the lenses can be worn during sleeping hours as well as waking hours over long periods of time, thinner, lower water content and thicker, higher water content (the proportion of water may range from about 40% to 90% by weight of the lens) lenses have been developed. Such lenses transmit sufficient oxygen by diffusion to supply the needs of the cornea when the eyes are open, even though they supply only marginal amounts of oxygen when the eyelids are closed. These lenses, however, rely on lens movement with consequent exchange of tear fluid to remove the corneal metabolic products. In addition, the great flexibility of current extended wear hydrogel lenses results in a closer corneal fit and greater difficulty in movement by motion of the eyelids. Consequently, removal of metabolic wastes remains a problem with extended wear lenses. Furthermore, even though the extended wear lens designs differ for low and high water content lenses, the oxygen transmissibility among lens types does not vary significantly (.+-.15%) from one type of lens to another. These lenses, in fact, transmit insufficient oxygen with consequent corneal hypoxia and build up of metabolic waste. These problems of inadequate oxygen transmission and poor movement have prevented the fulfillment of the promise of extended wear without metabolic complications in all existing hydrogel contact lenses, including those of the low minus-power type which are commonly prescribed for correction of the very common condition of slight to moderate myopia. Indeed, the relatively high frequency of corneal changes such as epithelial microcysts, corneal thinning, and occasional corneal ulcers associated with extended-wear contact lenses, and thought to be due to hypoxia has been described in Kenyon et al., "Influence of Wearing Schedule on Extended-Wear Complications", Ophthalmology 1986; 93:231-6. Lower water content contact lenses are presently optimized with respect to maximum oxygen transmission and cannot be improved upon utilizing existing technology. On the other hand, if the high water-content lenses are made thin enough to provide adequate oxygen to meet the metabolic needs of the cornea, other complications, such as epithelial erosions are observed. These effects are reported in Holden et al., "Epithelial Erosions Caused by Thin High Water Content Lenses", Clinical and Experimental Optometry 69.3: May 1986 --p. 103 -107, where it is suggested that these effects may be due to inadequate movement of the very thin high water contact lenses and desiccation of the cornea due to a very thin post-lens tear film.
Thus, contrary to thinking early in the development of soft contact lenses, problems in daily wear and extended wear lenses have recently been recognized. These problems include the effective disposal of metabolic wastes, anatomical alterations, and physiological alterations. The solution has been to design lenses having greater oxygen transmissibility characteristics by increasing the water content of the lens and/or by decreasing their thicknesses. Therefore, current extended wear lenses of low minus powers (-2.00 diopters) are known each with about equal oxygen transmissibility capabilities. For example, lower water content (35 -45%) lenses typically have a center thickness of 0.03 mm; medium water content (45 -59%) lenses typically have a center thickness of 0.05 mm; and higher water content (60 -90%) lenses typically have a center thickness of 0.15 mm. As discussed above, thinner lenses than those described of any water content have not been successful due to their fragility, poor handling characteristics, inadequate metabolic debris and waste removal properties, and the fact that they cause corneal damage.
Thus, a need has continued to exist for a high water-content hydrogel contact lens which combines adequate oxygen transmission with good movement over the cornea to prevent corneal damage, to remove metabolic debris and waste, and which handles well and is not too fragile.